Bright night lights, big science

In far southern and northern locations, such as Bear Lake (here) in Alaska, the night sky may be lit with the eerie green light of an aurora.

Trodel/Flickr (CC-BY 2.0)

In polar regions of the world, a dazzling light show often plays out in the night sky. It's called an aurora. Up North, it’s also known as the northern lights. It looks as if someone had stirred bright lights into the darkness, like cream into coffee. The lights are often green, but they also might glow red, yellow or other colors. Every so often, these lights explode in brightness. Such dramatic flare-ups are called auroral substorms. They produce a surge of intense light that travels westward. Now, two Japanese researchers have unraveled the physics behind these substorms. In short, they say: Blame the sun.

A stream of charged particles, called the solar wind, constantly flows out from the sun. When those particles collide with the Earth's magnetic field, they set off a cascade of collisions. That eventually leads to the light show.

The aurora first appears as a faint shine, usually only visible after the sun goes down. Substorms occur after the initial glow. To an observer on the ground, a substorm looks like the aurora gets brighter. Then sparks seem to travel westward.

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Yusuke Ebihara is the physicist at Kyoto University in Japan who led the new study. Working with Takashi Tanaka at Kyushu University, the two set out to better understand the details of those substorms.

They already knew that during an aurora an electric charge from the solar wind builds up on Earth’s night side. Tanaka used a computer to simulate the auroras. With this computer model, the physicists analyzed what must be happening in the upper atmosphere.

The electric charge can “overflow,” they found. It then sheds excess energy in the form of an electric current. That current can cause a brightening of the substorm. The researchers described their analyses December 21, 2015 in the Journal of Geophysical Research: Space Physics.

The new work shows that the electric current takes a different shape than previous models had predicted. “Our results are surprisingly different from what we believed previously,” says Ebihara. “I was surprised that the upper atmosphere participates actively in the formation of the bright aurora.”

Murphy studies auroras using ground-based cameras and other tools. He led an aurora study, published in December 2013. It used such observations to describe the currents that show up during a substorm. The findings from Tanaka and Ebihara match up with what he reported. “Their data agreed very nicely with our observations,” he says.

Ebihara notes that his team's new simulation helps explain why bright auroras appear or don't in certain solar-wind conditions. For example, his team’s computer model suggests that the surges don't form during the day because radiation from the sun is too strong. Yet many questions about the substorms remain unanswered, he notes. For instance, scientists still don't fully understand how the charged particles interact with Earth’s magnetic field.

Models like the one used by Ebihara are powerful tools, Murphy says. “They point us in the direction we need to go next.” In the future, he'd like to confirm predictions from models with actual observations. “That way, we can make sure we're not missing the physics.” If satellite images of the auroras line up with the models, he says, “then we know we've made the right conclusions.”

Power Words

aurora A light display in the sky caused when incoming energetic particles from the sun collide with gas molecules in a planet’s upper atmosphere. The best known of these is Earth’s aurora borealis, or northern lights. On some outer gas planets, like Jupiter and Saturn, the combination of a fast rate of rotation and strong magnetic field leads to high electrical currents in the upper atmosphere, above the planets’ poles. This, too, can cause auroral “light” shows in their upper atmosphere.

current (in physics) The flow of electricity or the amount of electricity moving through some point over a particular period of time. magnetic field An area of influence created by certain materials, called magnets, or by the movement of electric charges.

magnetic field An area of influence created by certain materials, called magnets, or by the movement of electric charges.

model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.

particle A minute amount of something.

physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study which emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in that field is known as a physicist.

satellite A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.

simulate To deceive in some way by imitating the form or function of something. A simulated dietary fat, for instance, may deceive the mouth that it has tasted a real fat because it has the same feel on the tongue — without having any calories. A simulated sense of touch may fool the brain into thinking a finger has touched something even though a hand may no longer exists and has been replaced by a synthetic limb. (in computing) To try and imitate the conditions, functions or appearance of something. Computer programs that do this are referred to as simulations.

solar wind A flow of charged particles (including atomic nuclei) that have been ejected from the surface of the star, such as our sun. It can permeate the solar system. This is called a stellar wind, when from a star other than the sun.

sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Or a sunlike star.

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